Ink jet printers are well known in the art, and are utilized in many different printing applications. For example, the metering/printing modules of many current mailing machines utilize ink jet printing technology to print evidence of postage, such as postal indicia that include a 2-D barcode. Generally, an ink jet printer includes one or more arrays of nozzles (sometimes referred to as orifices), a supply of ink, a plurality of ejection elements (for example, expanding vapor bubble elements or piezoelectric transducer elements) corresponding to the nozzles and suitable driver and control electronics for controlling the ejection elements. Typically, the one or more arrays of nozzles and the ejection elements along with their associated components are referred to as a print head. It is the activation of the ejection elements that causes drops of ink to be expelled from the nozzles. The ink ejected in this manner forms drops which travel along a flight path until they reach a print medium such as a sheet of paper, an envelope or the like. Once they reach the print medium, the drops dry and collectively form a print image. In addition, it is known that when drops of ink travel along a flight path after being ejected, they typically break up into a main drop and one or more smaller drops known as satellites, all of which land on the print medium. Typically, the ejection elements are selectively activated (energized) or not activated (not energized) to expel or not expel, respectively, drops of ink as relative movement is provided between the print head and the print medium so that a predetermined or desired print image is achieved.
In order to improve the operation and performance, various types of testing and analyses are performed on the ink jet print head and the images printed by an ink jet print head. Two important characteristics related to the operation of an ink jet print head are drop velocity and satellite velocity, which are the velocities at which drops and satellites, respectively, travel after being ejected from a nozzle of an ink jet print head. By studying operating characteristics of an ink jet print head, such as drop velocity and satellite velocity, print quality and print head performance can be improved by, for example, designing inks, nozzles and/or firing pulses in a manner that minimizes the production of satellites, adjusting the voltage applied to a nozzle to eject an ink drop, and choosing a particular well performing ink. In addition, it is often necessary to test the performance of a particular ink jet print head to determine whether it meets the requirements of a particular printing application.
Conventional equipment for measuring drop velocity and satellite velocity includes complex optical and electronic equipment that is very expensive and difficult to calibrate. Examples of such equipment are the Drop Watcher testing machine manufactured by Imaging Technology International Corporation of Boulder, Colo., and the Genie testing machine manufactured by VisionJet, a subsidiary of Xennia Technology Limited of the United Kingdom. The Genie product is a sophisticated laser based droplet measurement system that captures precise, quantitative data on a series of individual droplets, through which a thorough understanding of the performance and reliability of the ink jet print head may be determined.
Because of the high cost and complexity of these types of equipment, they are impractical to use for many applications. Thus, there is a need for a simple and inexpensive method and apparatus for testing the operation and performance of an ink jet print head.